1. Field of the Invention
The present invention relates to a process for producing a diffraction grating substrate for a semiconductor laser partially having a diffraction grating, by electron beam exposure or the like, as well as to a process for producing a semiconductor laser partially having a diffraction grating, using the above diffraction grating substrate.
2. Description of the Prior Art
In production of a semiconductor laser having a diffraction grating, it is one of the most important factors in ensuring the satisfactory performance and yield of the semiconductor laser to precisely control the diffraction grating shape of the diffraction grating substrate used in the semiconductor laser. In order to attain the precise control, it is conducted to form, on a semiconductor substrate by electron beam exposure (hereinafter referred to as EB exposure), a fine resist pattern of diffraction grating having precisely controlled aperture intervals. And then the semiconductor substrate is etched to transfer the resist pattern onto the substrate. In this case, formation of a diffraction grating pattern on the whole surface of a wafer by EB exposure takes an enormous exposure time; therefore, it is ordinarily conducted to form a diffraction grating pattern on part of a wafer and only the pattern region is subjected to EB exposure to form an aperture pattern.
FIG. 13 is a schematic drawing of a diffraction grating substrate in which a diffraction grating has been formed by a conventional process. This structure is formed as follows. A resist 305 is coated on a semiconductor substrate 302; at part of the coated resist, a resist pattern 301 for formation of diffraction grating is formed; then, wet etching is conducted to form a diffraction grating. As shown in FIG. 13, when the resist pattern is a repetition of an aperture of given area, the etching rate become larger in the vicinity of the end 303 of resist pattern formation region owing to a pattern effect; as a result, overetched hollowing 304 of diffraction grating takes place in the vicinity of said end, making it impossible to form a diffraction grating of uniform shape.
In a semiconductor laser partially having such a diffraction grating of nonuniform shape, there occur, at the end of diffraction grating formation region, light reflection and scattering caused by the overetched hollowing of diffraction grating, and which brings on laser beam waveguide loss and nonuniform field distribution.